1. Field of the Invention
The present invention relates to actuators. More particularly, the present invention relates to rotary actuators of the type which produce rotary motion of a shaft by means of a piston oscillating in a generally arcuate chamber.
2. Description of the Background
Rotary actuators are used in a variety of applications where it is desired to effect movement of a rotary fashion about a center point. For example, such actuators can be used to open and close valves, turns switches, operate steering mechanisms, etc. The actuator may be of the double-acting type wherein fluid, either hydraulic or pneumatic, is used to displace the piston in an oscillating manner in an arcuate chamber to hence effect rotation of a shaft in a clockwise or counterclockwise direction, depending upon movement of the attached piston. Alternatively, rotary actuators can be of the single-acting type wherein the fluid pressure is used to displace the piston and hence effect rotation of the shaft in one direction while rotation of the shaft in the other direction is accomplished by mechanical means, such as a spring return which, upon release of the pressure acting on the piston, automatically returns the piston to its initial position. In the rotary actuator described in U.S. patent application Ser. No. 452,207, incorporated herein by reference for all purposes, and in the preferred embodiment disclosed therein, the piston is of a type which has a generally elliptical, cross-sectional configuration. The use of a piston having that particular shape of cross section results in greater piston area than can be achieved with a piston having a circular cross section and thereby results in a corresponding increase in torque. At the same time, the overall diameter and "foot print" of the actuator is not increased. Moreover, the elliptical or oval design is clearly preferable to a rectangular design because of the difficulty in attempting to effect a fluid-tight seal between a piston having such a cross section and the housing in which the piston moves. Although there are many advantages to using a piston with a generally elliptical or obround, cross-sectional configuration, problems can arise in sealing around the periphery of the piston. It is known that in rotary actuator, as the piston oscillates in the housing, there is a tendency for the piston to undergo deflection making it difficult for the floating seal to maintain sealing engagement with the piston. In particular, in the case of pistons having obround or oval cross-sectional configurations, the seal deflection is not uniform but is more accentuated at the elongated sides of the piston with the result that the seal has a tendency to pull away from the elongate sides permitting leakage of the fluid pressure past the piston.
In an attempt to overcome that problem, and as disclosed in application Ser. No. 452,207, a unique seal assembly was used which included a generally flexible, taut band in surrounding relationship with the seal ring, the band serving to prevent outward bowing of the seal ring away from the elongate sides of the piston.